R&B star Usher's 11-year-old stepson has been declared brain dead by doctors following a jet-ski accident on Saturday.

Kyle Glover, the son of the singer's estranged wife Tameka Foster, was sitting on an inflatable raft which collided with the watercraft on Lake Lanier in Atlanta, Georgia.

He suffered head injuries and was taken to hospital in a critical condition. Glover has shown no brain activity since the accident, according to TMZ.com.

As WENN went to press a decision had not been made on whether to take Glover off life support.

Usher reportedly chartered a private jet for Foster, who was out of town at the time of the accident, so she could race to her son's bedside.

Meanwhile, officials in Georgia have launched an investigation into the jet ski accident.

Robin Hill, a representative for Department of Natural Resources, tells TMZ.com the department's Critical Incident Response Team will recreate the accident using special computer programmes to determine how fast the jet-skier was travelling at the time of the accident.

The findings will be turned over to the district attorney, who will determine whether or not to file charges.